1. Field of the Invention
The present invention relates to a voltage controlled oscillator that constructs a phase synchronous circuit for use in the OSD display control for television screens and the like, and more specifically, to a voltage controlled oscillator that controls the variations of an oscillation frequency to the variations of a power supply.
2. Description of Related Art
FIG. 4 is a block circuit diagram showing a conventional voltage controlled oscillator (hereinafter, abbreviated to as VCO) that constitutes a phase synchronous circuit for use in the OSD display control. In FIG. 4, reference numeral 101 designates a voltage-current conversion circuit; 102 designates a ring oscillator; 103 designates a main power supply for feeding a power supply voltage Vcc to these voltage-current conversion circuit 101 and ring oscillator 102; Vc denotes a VCO control voltage for controlling the voltage controlled oscillator; Vout denotes an oscillation output from the voltage controlled oscillator; and 110 designates ground. Note that a control signal generated by the voltage-current conversion circuit 101 is designated by Vic.
The operation will be next described below.
The VCO control voltage Vc is inputted to the voltage-current conversion circuit 101, which converts the power supply voltage Vcc to generate the control signal Vic. The ring oscillator 102 is controlled by the control signal Vic to generate the oscillation output Vout. In other words, these construct a voltage controlled oscillator in which the oscillation output Vout is controlled by the VCO control voltage Vc. In addition, the oscillation output Vout becomes a signal to generate the VCO control voltage Vc, and also serves as one type of loop circuit.
Since the conventional voltage controlled oscillator is configured as described above, the oscillation output Vout is varied or is not put in a locked status due to the variations of the power supply voltage Vcc even when the VCO control voltage is constant in the phase synchronous circuit such as PLL circuits having such a voltage controlled oscillator. Thus, when it is employed for the OSD display control, there are some problems such that swinging literatures and jitters occur on the display.
The present invention is implemented to solve the foregoing drawbacks. It is therefor an object of the present invention to provide a voltage controlled oscillator capable of maintaining the locked status with controlling the variations of the oscillation output to that of the power supply voltage.
According to a first aspect of the present invention, there is provided a voltage controlled oscillator comprising: a main power supply for feeding a power supply voltage; a regulator circuit for, in response to the power supply voltage, generating an internal voltage with controlling the variations of the power supply voltage; a voltage-current conversion circuit for generating a control signal based on an internal voltage fed from the regulator circuit by inputting a control voltage from the external; and a ring oscillator for generating an oscillation output based on the internal voltage fed from the regulator circuit by inputting the control signal.
According to a second aspect of the present invention, there is provided a voltage controlled oscillator comprising: a main power supply for feeding a power supply voltage; a regulator circuit for, in response to the power supply voltage, generating an internal voltage with controlling the variations of the power supply voltage; a voltage-current conversion circuit for generating a control signal based on the power supply voltage fed from the the main power supply by inputting a control voltage from the external; and a ring oscillator for generating an oscillation output based on the internal voltage fed from the regulator circuit by inputting the control signal.
Here, the voltage controlled oscillator may further comprise: a switch arranged between the regulator circuit and the voltage-current conversion circuit to be capable of connecting the main power supply with the voltage-current conversion circuit, while bypassing the regulator circuit, thereby selecting and feeding either the internal voltage from the regulator circuit or the power supply voltage from the main power supply.